By default, Magento keeps separate counts for order and invoice numbers. This extension ensures that when an invoice, credit memo or shipment is created, it uses the related order number.
Once installed, new invoices / credit memos / shipments will always take the same number as the associated order – eg. if you invoice order 1000, then the invoice, any shipment and any credit memo would also be named 1000. In the event of multiple invoices/shipments/credit memos, a duplicate separator will be used – eg. invoice 1000-1, 1000-2, 1000-3 and so on.
The extension does not change any historic data – only future invoices / credit memos / shipments will be changed.